1. Field of the Invention
The present invention relates generally to printing devices such as typewriters and printers and more particularly to means for detachably coupling a typewriter or printer printwheel with a typewriter or printer drive plate. According to the present invention, the radial and angular position of the printwheel with respect to the drive plate is maintained in alignment and relative movement between the printwheel and the drive plate is inhibited.
2. Description of the Prior Art
Printwheels used in typewriters and printers ("printing devices") are generally affixed in such a manner as to be readily and easily removed and replaced in order to change the printwheel or, in some products in which the printwheel must be removed before changing the ribbon cartridge, to remove and replace the ribbon cartridge. It is desirable that the structure for affixing the printwheel should be simple, reliable and capable of being manufactured at low cost.
Attempts to achieve these characteristics have included, for example, the printing device disclosed in U.S. Pat. No. 4,556,335 which includes a printwheel centered on a printwheel drive shaft by an operator and prevented from slipping relative to the shaft by the spring-loaded retention of pivot posts in the channels of a printwheel connector. The printer disclosed in U.S. Pat. No. 4,542,999 includes a resilient "Z" shaped arm on a printwheel seated in a bracket by an operator which provides securement between the printwheel and the bracket by means of a wedgelike engagement in a recess formed in a bracket.
According to the present invention a hubbed printwheel has a recess formed with a radially offset cam surface. The drive plate associated with the printing device includes an alignment pin which engages the offset cam surface on the printwheel for inhibiting radial and angular movement of the printwheel relative to the drive plate. Smith Corona Corporation, the assignee of the present application, has sold prior art printing devices, such as its typewriter model XL 1700, which have included a hubbed printwheel having a recess formed with an offset cam surface.
The drive plates associated with the Smith Corona devices do not, however, include an alignment pin engaging the radial offset cam surface on a printwheel, nor do the drive plates include any structure for automatically engaging an alignment pin with a radial offset cam surface.